There has been known a hydraulic drive system for a work machine having a regeneration circuit by which hydraulic fluid discharged from a boom cylinder due to falling of a boom by its own weight is regenerated, for example, for an arm cylinder, and examples thereof are described in Patent Documents 1 and 2. In the hydraulic drive system described in Patent Document 1, at the time of regeneration of the hydraulic fluid discharged from a bottom-side hydraulic chamber of the boom cylinder for the arm cylinder, delivery flow rate of a hydraulic pump for supplying the hydraulic fluid to the arm cylinder is reduced by an amount according to the regeneration, so as to improve the fuel cost for an engine.
Besides, in the hydraulic drive system described in Patent Document 2, the hydraulic fluid discharged from a bottom-side hydraulic chamber of the boom cylinder is regenerated for the arm cylinder through a center bypass line on the basis of judgment that a predetermined condition is established, whereby a hydraulic circuit is prevented from becoming large in size or complicated in structure.